Polarization transformation and electro-optic effects have been widely explored as physical foundations for optical devices. Optical MEMS devices, many involving microscopic mirrors, have also been the subject of intense development over the last fifteen years. However micro- or nano-fabricated devices that change the polarization of light and induce effects similar to electro-optical phenomena are not well known. An understanding of polarization and how it may be expressed in terms of linear combinations of orthogonal polarization states is necessary to understand polarization-based NOEMS or optical MEMS.
Linearly polarized light is characterized by an electric field vector of constant orientation. In contrast, unpolarized light has such a rapidly varying succession of different electric field orientations that no single polarization state is discernable. Unpolarized light may be represented as a superposition of equal-amplitude, incoherent, orthogonal, linearly polarized fields. A linear polarizer is a device that separates these two components. Circular polarization is a superposition of equal-amplitude, linearly polarized fields with a relative phase delay of π/2.
It is possible to separate a light beam into orthogonal polarization components, impart a phase delay upon one of the components, and then reassemble the components into a single beam. Operations of this type transform the polarization of the light beam. For example, if a linearly polarized beam is separated into two equal-amplitude, orthogonal polarization components, one of the components is phase delayed by π/2, and the phase shifted components are recombined, then the resulting polarization of the reassembled beam is circular polarization. A phase delay of π transforms the polarization to linear polarization oriented perpendicular to the original polarization.
Birefringent wave plates are commonly used to change the polarization of an incident light beam by imparting a relative phase delay upon polarization components of the beam. In a birefringent wave plate, the index of refraction is not isotropic; it varies depending upon the orientation of the plate. When light passes through a birefringent wave plate, polarization components of the light are subjected to unequal indices of refraction and therefore experience a relative phase delay.
It is possible to temporarily induce birefringence in electro-optic materials by applying an electric field. In the Kerr effect and the Pockels effect, an isotropic, transparent substance takes on the characteristics of a uniaxial birefringent crystal in the presence of an electric field. The difference in index of refraction in directions parallel and perpendicular to the electric field is proportional to the field in the Pockels effect and to the square of the field in the Kerr effect. An example of an electro-optic device is a Kerr shutter which is made by placing a Kerr cell (i.e. a suitable substance across which an electric field is applied) between crossed linear polarizers. When no field is applied, no light is transmitted as the polarizers are crossed. The application of a modulating voltage generates a field causing the cell to function as a variable wave plate and thus opening the shutter proportionately. Examples of electro-optic materials include carbon disulfide (liquid) and barium titanate (solid).